The use of semiconductor materials in electronics, photovoltaics, illumination and other applications has rapidly developed in recent decades, and continues to grow exponentially. Further growth of these industries is greatly dependent on the ability to develop cost-effective production of semiconductor materials. Epitaxy is one method of producing semiconductors, in which crystalline layers are deposited on a substrate. In homoepitaxy, the crystalline layer is grown on a substrate of the same material. In heteroepitaxy the crystalline film is grown on a different material, which allows for more readily-available materials to be used as substrates, and also allows for layers of different materials to be integrated together. Deposition of the layers typically occurs by vapor phase epitaxy, in which the crystal layers are deposited under chemical reaction, usually at relatively high temperatures. Although silicon has long been used as a semiconductor substrate, advancement of materials and technologies is requiring development of other choices for substrates. Selection of substrate materials depends on many factors, such as electrical properties, thermal properties, crystalline compatibility with the deposited layers, and cost.
One application of heteroepitaxy is in light emitting diodes (LEDs), such as LEDs formed from gallium nitride (GaN) grown over sapphire. Silicon carbide (SiC) and gallium arsenide (GaAs) are other substrates used in LEDs. GaN is capable of efficient light emission from deep ultraviolet to infrared wavelengths, and thus is a key material being developed for semiconductor-based white light sources. Choices for other materials in LED are based on factors such as cost, and compatibility with processing steps in the fabrication process.
Another application of heteroepitaxy is in transistor devices. Solid-state power devices—used in switching or amplifying large voltages and currents—are important components in communications, power delivery, and increasingly, transportation applications. One of the biggest innovations in this field in the last ten years has been the introduction of high electron mobility transistors (HEMTs) made on III-V semiconductors such as gallium nitride. HEMTs are devices that utilize a heterojunction between materials of differing bandgaps, where a two-dimensional electron gas (2DEG) is formed at the junction. The electrons have higher mobility at this heterojunction compared to moving through a doped region as in other semiconductor devices. HEMTs can be used at higher frequencies, control larger voltages in smaller areas, and dissipate (that is, waste) less power than similar transistors made with silicon. However, HEMTs face similar materials and processing challenges as LEDs.
Thus, there is an increasing need to produce heteroepitaxial materials with efficient cost and manufacturability.